How much gold is in your rock?

Two weights on a kitchen scale tell you the gold content of a nugget or specimen.

1. Weigh it dry Nugget straight on the scale.

2. Weigh it in water Put a cup of water on the scale, press tare (zero), then lower the nugget in on a thread so it hangs in the water without touching the cup. Read the scale.

Dry weight (grams)

Water reading (grams)

The scale reading with the nugget hanging in the tared water

Rock type in the specimen

Quartz is by far the most common in Victoria

–g gold

–gold by weight

–karat equivalent

–g rock

How this is worked out

The water reading is the weight of water the nugget pushes aside, which equals its volume in cm³. Gold is very dense (SG 19.3) and rock is not, so the volume tells you how the weight is split.

Treat the answer as a good estimate, not an exact figure. Three things move it: the sum assumes pure gold, and natural gold is usually 92–97% fine, so the true gold is a few percent lower than shown; any trapped air or a vug in the specimen reads as extra rock; and on a small piece the accuracy of your scale dominates everything — 0.1 g of slop in the water reading can move the result by a gram or more. Weigh it two or three times and see how much the answer wanders.

DetectorMods, Ballarat. Every figure here is an estimate from two weights — assaying is the only exact method, and the only one a buyer will settle on.